The present invention relates to reference signal generation for lock-in amplifier in high sensitivity gas sensing systems.
High sensitivity optical based gas sensing system detects the absorption of a modulated optical signal after passing it through the gas path. The optical signal is generated from a tunable laser whose wavelength changes according to the amplitude of its driving power. The modulating signal includes a first periodic signal (frequency fα=2πα) for wavelength tuning, and a second periodic signal (frequency fθ=2πθ) for modulation. The first periodic signal is usually a ramp (or a saw-tooth signal) for wavelength tuning; the second periodic signal is mostly a sinusoid that has frequency much higher than the first signal. The detector uses lock-in amplifier to remove most of the noise while amplifies the signal.
A lock-in amplifier requires a local reference signal that has the same frequency and phase as the second periodic modulation signal (i.e., frequency 2πθ). This signal is usually generated through a phase-locked-loop (PLL). The lock-in amplifier converts the signal to DC and uses a very-narrow-band low-pass filter to filter out noise.
FIG. 1 shows a block diagram of a gas sensing system, which includes a signal generator 120, gas-absorption path 106, and signal detector 122. Signal generator 120 further includes electrical signal generator 102, laser and its driving circuit 104. Laser wavelength changes in accordance with the amplitude of its modulation signal. Electrical signal includes a first periodic signal s1, which serves as the main wavelength scanning signal; a second periodic signal s2 with frequency 2πθ, which is usually sinusoidal and works as a modulation signal; and a DC signal to set the proper bias current for the laser. Optical path 106 has spectral-selective absorption for the selected wavelength in accordance with the gas of interest. Detector system includes a photo-detector and pre-amplifier 108, an optional band-pass filter with central frequency 4πθ, reference signal 112, multiplier 114, and a low-pass filter 116. Reference signal 112 has frequency of 4πθ, to detect the second harmonic of the absorbance. A base band output is generated from low-pass filter 116 and carries gas concentration information.
In a gas sensing system, for higher sensitivity and laser fluctuation tolerance, second order demodulation is used which picks the second harmonic (i.e., 2× frequency which is 4πθ) of the second modulation signal from the absorbed signal, which is the second derivative of the absorption. For such case the local reference signal needs to be 2× the frequency and 90° phase shifted to the second modulation signal. A simple PLL at the second harmonic, in particular in analog circuit, does not work well since the amplitude of the received signal at the second harmonic frequency varies depends on gas density, and can be approximately zero when the gas density (thus the absorption) is extremely low. This may mean “loss of signal” to the PLL. Further, the demodulated signal at the second harmonic frequency is not DC but the second derivative of the absorption. A simple PLL may not converge well even when the signal is not too small.
In addition, a PLL usually converges slowly. For a detector that receives signals from different paths, these signals are unlikely to be aligned, so each time to switch from one path to another, it takes time for the local reference signal to get locked.
Due to the usage of a first signal for wavelength scanning, and the detection of the second harmonic which results in very small signal, a ultra-low-jitter local reference is needed while a traditional phase-locked-loop (PLL) cannot be used. In addition, for application that a single sensing system (in terms of signal generation and detection) is used for multiple sensing paths, which leads to phase differences of the detected signal, a traditional PLL usually takes time to converge whenever switches from one path to another, which greatly reduces system measuring frequency.